Spread-spectrum communication systems characteristically spread the spectrum of the signals they transmit over a bandwidth which greatly exceeds the information bandwidth of the signals, As a general rule, communication systems of this type are designed to distribute the transmitted signal energy more or less uniformly throughout a relatively broad band of frequencies, so relatively little of the transmitted signal energy resides within any given narrow frequency band.
Spread-spectrum communication systems offer several advantages over conventional narrow band systems. For example, spread-spectrum signals are relatively immune to interference from, and are less likely to cause interference to, narrow band ("nonspread") signals. Furthermore, some spread-spectrum systems are compatible with the use of code-division multiplexing for carrying out multiple communications independently at the same time and frequency. As is known, code-division multiplexing is a convenient alternative to time division and frequency division multiplexing for sharing the spectrum among multiple users. In view of these advantages, it is believed that spread-spectrum communications will become more commonplace, especially within the license-free bands (i.e., 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz) which have been allocated for transmissions of this type as transmitted power levels of up to 1 watt, subject to certain restrictions relating to the distribution of the sideband energy.
Several techniques have been developed for performing the signal spreading for spread-spectrum communications, including "frequency hopping" where the center frequency of a RF carrier is cyclically varied at a relatively high rate in accordance with a predetermined table of frequencies, and "direct-sequence" spreading where the phase of the RF carrier is rapidly varied in accordance with a pseudo-random binary sequence or "code."
In a direct-sequence spread-spectrum communication system, the transmitter conventionally mixes a pseudo-random code-like sequence with an information modulated carrier-signal, thereby distributing the signal energy throughout the available bandwidth. Any of a number of well known modulations techniques may e employed for impressing data or other information on the carrier. The phase modulation produced by the pseudo-random sequences causes the spectral density of the transmitted signal to be uniform to a first approximation across a wide band of frequencies.
Straightforward digital circuitry is available for generating such pseudo-random sequences. However, improved methods and means still are needed for "de-spreading" the spectrum spreaded signal at the receiver so that the carrier can be recovered from it. To carry out this de-spreading function, the spread-spectrum signal appearing at the input of the receiver must be mixed with a pseudo-random sequence which is identical in both frequency and phase to the signal spreading sequence employed at the transmitter. This process commonly is referred to as "synchronization."
Several synchronization techniques have been developed for direct-sequence spread-spectrum communications. See, for example, R. C. Dixon, Spread-Spectrum Systems, 2 Ed., Chapter 6, John C. Wiley & Sons, 1984. Some of the approaches that have been proposed are very complicated, or rely on special spreading codes for enhancing synchronization. For instance, there are proposals which require the receiver to have multiple correlators operating in parallel in order to find the proper code phase for de-spreading the received signal. Thus, in the interest of simplifying this disclosure, the following discussion concentrates on the known synchronization techniques which are believed to be most relevant to this invention.
One of the simpler of the available synchronization techniques is referred to as the "transmitted reference" method. This type of synchronization often is carried out by employing the same pseudo-random sequence for spreading the spectra of two carriers. The same information is impressed on both of these carriers, but the carrier center frequencies are sufficiently widely separated that there is no significant overlap between their sidebands in the frequency domain. Therefore, a carrier can be recovered at the receiver by mixing the outputs of a pair of tuned amplifiers, each of which is tuned to the spread-spectrum of a respective one of the transmitted carriers. The frequency of the carrier which is recovered by the mixer is equal to the difference between the frequencies of the transmitted carriers, so the information content of the transmitted carriers is preserved because it is a "common mode" signal (i.e., common to both of the carriers). As will be appreciated, the receiver does not require any dedicated synchronization circuitry to carry out this synchronization process. However, that advantage is offset by significant disadvantages, including the susceptibility of the receiver to interference from other transmitters, and the lack of knowledge at the receiver of the transmitted code. Such knowledge is, of course, necessary if it is desired to employ code-division multiplexing.
There also are prior so-called "carrier lock tracking" synchronization techniques for direct-sequences spread-spectrum communication systems. To carry out this type of synchronization, the carrier is synchronized with the code clock at the transmitter, and a sliding correlator is employed within the receiver to search for the proper code phase. The sliding correlator characteristically comprises a pseudo-random code generator for driving a mixer to which the incoming spread-spectrum signal is applied. This code generator is matched to the transmitter code generator, so that it can substantially replicate the transmitted code sequence when it is operating in phase synchronism with the transmitter code generator. When, however, the receiver is operating in an idle or standby mode, its code generator is driven by a local clock source at a frequency which is offset slightly from the clock rate at which the transmitter code generator is driven, whereby the relative phase of their respective code sequences varies sufficiently slowly that correlation can be detected, typically in the time it takes for their relative phase to slip by one bit. Once correlation is detected, the receiver code clock switches over to the transmitted code rate, either by switching the receiver code generator so that it is clocked by a separate local clock which is preset to the same frequency as the transmitter code clock or by phase locking the receiver code clock to the carrier. Thus, in these known carrier lock tracking-type synchronizers, the sliding correlator is used to find lock, and carrier detection then is employed to maintain lock. If the spectrum spreading code sequence is of moderate length, lock can be obtained in a reasonable amount of time. The receiver essentially replicates the transmitted code sequence when it is locked to the transmitter, so code-division multiplexing can be employed for carrying out simultaneous communications between several different transmitter/receiver combinations over the same band of frequencies.
Known carrier lock synchronization techniques customarily tie the code clock frequency (i.e., the "code rate") to the carrier frequency, thereby restricting the designer's freedom of choice with respect to one or the other of those frequencies. As a general rule, the carrier frequency is significantly higher than the code rate, so a frequency divider typically is employed in the receiver for deriving the code rate from the carrier, even though this may create a phase ambiguity which then has to be resolved by the correlator. For example, if the carrier frequency is n times higher than the code rate (where n is an integer greater than one), n phases of the carrier will produce code phases within a one-bit wide correlation window, so the correlator then typically is required to determined which code phase provides the strongest carrier output for the best signal to noise ratio.
Others have attempted to reduce this code-phase ambiguity by augmenting the receiver with a local oscillator which is driven by a frequency multiplier at a frequency which is derived from the local code clock. In these systems, a mixer mixes the spread-spectrum signal that is received by the receiver with the frequency generated by the local oscillator, such that the carrier frequency is shifted to (n-m) f.sub.c, where f.sub.c is the carrier frequency of the transmitter, and m is the integer factor by which the frequency multiplier multiples the receiver code clock frequency. Consequently, if (n-m) is selected to equal one, the frequency of the de-spread carrier is equal to the code clock rate or frequency, f.sub.ck. Even then, however, there still may be an ambiguity caused by having two code phases within the one-bit wide correlation window because the relative phase of the transmitter and receiver code clocks is unknown and not easily predictable. See, Dixon, supra, at pp. 254-257.
Power consumption is another important consideration, especially for spread-spectrum communications between portable, battery powered stations, such as computers. In these applications, the receiver is likely to be powered-up continuously, so the amount of power it draws can be critical. Unfortunately, the frequency dividers employed in prior carrier lock tracking synchronization schemes tend to draw a substantial amount of power, especially when the communications are being carried out at very high frequencies, such as within one of the aforementioned license-free bands. At the current state of the art, emitter coupler logic (ECL) typically is required for frequency dividers operating at such high frequencies, but logic of that type is ill-suited for applications requiring power conservation. Furthermore, high signal levels for driving ECL dividers, so the amplifiers that are needed to provide such signal levels consume additional power. Therefore, there is a need for an economical and easily implemented lower power, code-division multiplexing compatible, synchronization technique for spread-spectrum communications.